1. Field of the Invention
The invention relates to zinc salts of sulfonic acids and their use in diazotypy.
2. Brief Description of the Prior Art
The diazotype reproduction process is well-known to the art and is described in great detail in "Light Sensitive Systems" by Jaromir Kosar, John Wiley & Sons, Inc., N.Y. 1965, and in "Reproduction Coating" by E. Jahoda, 4th Edition, Andrews Paper & Chemical Co., Inc., Port Washington, N.Y.
In general, diazotype reproduction prints are obtained by an imagewise exposure of a light-sensitive diazo material to ultraviolet light followed by development of the exposed diazotype material. There are several different methods available to develop the latent image imposed on the light-sensitive diazo material by the light exposure including ammonia development, amine development, thermal development and moist (liquid) development.
The light-sensitive diazomaterial generally comprises a light-sensitive diazo composition affixed to a support base such as paper or film such as polyethylene terephthalate film, cellulose acetate, or a polymeric resin coated, like base support. The light-sensitive diazo composition may comprise a light-sensitive diazonium salt compound in admixture with diazo enhancing compounds. Upon exposure of the light-sensitive diazonium compound to ultraviolet light through a translucent original having opaque image portions, the unmasked portions of the diazonium salt are decomposed by the ultraviolet radiation whereas the masked portions are left undecomposed. The latent image created by the image-wise exposure may then be developed by the methods described above.
In the so-called "dry development" process, the light-sensitive diazo composition will contain, in addition to the light-sensitive diazonium salt, an azo coupling agent or color former and an acidic coupling inhibitor. Development of the latent azo dye image is accomplished by placing the exposed diazo material in an alkaline atmosphere which neutralizes the acidic inhibitor, allowing the undecomposed diazonium salt and coupler to react. The latent image is thereby developed.
In the moist development process (also called the one-component process) the light sensitive diazo composition contains essentially the diazonium salt compound. After image-wise exposure, development of the latent azo dye image is accomplished in a pH adjusted solution containing at least one coupler to react with the undecomposed diazonium salt.
It is of great importance in both of the above described processes that print development is obtained rapidly and that a minimum of premature development and other deterioration of diazotype materials occur during the time between their manufacture and their use.
Unfortunately, most diazo compounds have low stability and tend to decompose quickly. The coupling reaction unavoidably begins and continues at a very slow rate, immediately after the manufacture of the diazotype materials (i.e., precoupling). Consequently, their shelf life is very limited unless efficient stabilizers are added to delay or block such reactions.
Previously known stabilizers for diazotype materials have several drawbacks. For example, they tend to slow the coupling reaction even during development when it is supposed to occur rapidly. They also may adversely influence the developed image appearance by reducing brilliance and shifting color shades. Many stabilizers are not compatible with diazo coating solutions because they rapidly form insoluble salts or complexes with diazo compounds or couplers.
Conventional stabilizers used for diazotype materials are generally acids, such as boric acid, citric acid, tartaric acid and the like or inorganic acid salts such as zinc sulfate, aluminum sulfate, zinc chloride and the like. The free acids, in relatively low concentrations tend to slow the rate of development by lowering the pH. The stronger free acids render many conventional coated substrates brittle.
Zinc chloride is among the most widely used of stabilizers in diazotype materials. Its presence stabilizes diazo compounds against decomposition and is very effective against precoupling. However, the concentration of zinc chloride required for stabilization has many drawbacks. For example, it tends to precipitate most diazos as well as many couplers from their solutions. Its presence also inhibits print surface wetting by the developer solution in the moist processes. Its presence may also inhibit coupling of diazotypes in amine type of development and in the thermal type development processes. It may also render the diazo coating layer tacky because of its pronounced hygroscopic character.
Alkali salts of aromatic sulfonic acids are very effective stabilizers for diazo materials despite their being close to a neutral pH since they form sulfonic acid salts with the diazonium ion. However, these diazonium salts often have very limited solubility and thus precipitate from the sensitizing solution. Another drawback of alkali salts of aromatic sulfonic acids is that their presence slows the rate of image development considerably.
The present invention obviates many of the prior art problems and provides diazotype materials having good shelf life and a faster rate of image development. These objectives are accomplished by employing a new class of stabilizers having improved compatibility with diazotype sensitizing solutions. They do not unduly inhibit print dye coupling during the image development process. The invention revolves around the discovery that zinc salts of sulfonic acids can be used to stabilize diazotype coatings with particular advantages over prior art stabilizers. They are suprisingly compatible with diazotype coating solutions. Higher concentrations of stabilizer may be employed for better stabilization. They also do not inhibit the wetting of the print surface by the developer solution in the case of moist developed diazotype processes, do not inhibit amine development, and provide coating layers that are much less hygroscopic and thus pass more easily through hot ammonia developing machines.
In comparison to alkali salts of sulfonic acids, zinc salts of sulfonic acids are superior diazo stabilizers because they provide better diazotype coating solution compatibility, faster rate of development, and better print background stability (preventing discoloration).
Other advantages of the present invention include improved continuous coating coverage. Print exposure latitude may be improved through complete or partial replacement of zinc chloride stabilizer with zinc salts of sulfonic acids.
Another advantage is that the presence of the zinc salts of sulfonic acids as stabilizers provide more neutral (less pink) shades in blackline diazotypes.
Still another advantage is found in that presence of zinc salts of sulfonic acids in diazotype materials used for ammonia development processes provides print colors that are more stable to pH variations and their hues change much less between development and the time when all excess ammonia has been dissipated. This improved pH stability of prints is of particular importance with blackline and redline diazotype materials.
Another advantage discovered is that the use of zinc salts of sulfonic acid as a stabilizer instead of zinc chloride or 1, 3, 6-1, 3, 7-naphthalene trisulfonic acid in diazotypes for amine development spectacularly increases the rate of image development while providing for good shelf life stability.
We found, moreover, the zinc salts of sulfonic acids are soluble in polaric organic solvents and compatible with polymeric resins such as cellulosic esters and that they are excellent stabilizers for diazotype film coating compositions, not exuding from the coating layer upon print ageing.
While the reasons for the above and other beneficial effects of the zinc salts of sulfonic acids are not fully understood, it is believed that the polaric structure of the zinc salts of sulfonic acids of this invention have a solubilizing effect that improves coating solution stability and wetting out of the precoated or non-precoated base paper surface with the sensitizing solution. Also, the simultaneous presence of zinc ions and sulfonic acid ions seem to have a synergistic effect with respect to improving shelf life stability against precoupling and diazo decomposition, without inhibiting the coupling rate during development. Moreover, the almost neutral but very slightly acidic pH of the zinc salts of sulfonic acids of the invention improves buffer capacity, minimizing print background discoloration when prints are kept under alkaline or acidic atmospheric conditions.